The present invention is generally directed to a method and apparatus for producing via holes in polymer dielectrics without the use of a mask. More particularly the present invention is directed to producing via holes in a dielectric film which is deposited over integrated circuit chips, the film operating as an insulative support medium for conductive material which interconnects the contact pads on the same and/or different chips where the chips may exist within a wafer or be separate and supported on a substrate. The conductive material takes the form of a patterned metal layer which overlaps and fills the holes in the film. The method and apparatus of the present invention produces via holes in a polymer film overlay and thereby provides a means for electrically interconnecting parts of one or a plurality of circuit chips disposed on a substrate through the vias thus formed as specifically provided in copending and commonly assigned patent application Ser. No. 947,151, (RD-17193), filed in the name of Alexander J. Yerman and Constantine A. Neugebauer, entitled "Fabrication of Large Power Semiconductor Composite By Wafer Interconnection of Individual Devices". It is also noted that the present invention provides significant advantages in a system of microchip packaging.
Polymer dielectrics are finding increased use in multichip packaging approaches because such dielectrics are easily applied at low tempertures and result in relatively thick coatings having a low dielectric constant. More particularly, the problem addressed by the present invention is the production of holes in such polymer layers for the purpose of connecting metallization on the top of the polymer to metallization under the polymer dielectric.
One prior art method for providing such via holes in a polymer is to apply a metal mask to the top surface of the polymer by metal deposition. For example, a 1,000 Angstrom thick layer of titanium can be applied. The titanium is then processed by photolithographic methods and holes are etched in the titanium where via holes are desired. The polymer is then etched in an oxygen plasma. The oxygen plasma does not attack the titanium, but does attack the exposed polymer. One main disadvantage of this technique is that it involves a substantial number of steps which add greatly to the complexity and expense of process: depositing the metal mask which involves first cleaning the polymer for good adhesion then depositing a photoresist, drying the resist, exposing the resist, developing the resist, hard-baking the resist, then etching holes in the metal mask. Secondly, these patterning steps involve the use of masks which are not easily changed if such changes are necessary due to changes in the circuits being fabricated. This is followed by a carefully controlled plasma etch step which is highly dependent on the temperature of the etchant and gas pressures. Additionally, the metal mask layer must be removed in order to assure good adhesion between the conductor metallization which is to be applied next and the polymer.
An alternative approach to forming via openings is to spin or spray polyimide on a substrate and only partially cure the polyimide. Subsequently, the polyimide is coated with a photoresist and the resist is developed. In the partially cured state, the polyimide is also attacked by the developer and via holes can be etched in thin films of polyimide. This process is not satisfactory for thick films of polyimide since entrapped water vapor in the polymer cannot escape. The limit on this process is a thickness of 5 microns. In addition, this process could not be used to produce an overlay layer across the space between two chips since there is no supportive film involved in spraying or spin methods. Photosensitive polyimides are becoming available, but they suffer the same problems of thickness and inability to provide a continuous film across two chips.
A method which can be used to provide via openings through relatively thick layers of polymer involves patterning the lower layer of metallization and building up by electroplating the areas where vias are desired. This essentially leaves pillars of conductor material where the via is desired. Polymer material is then sprayed or spun on the substrate in multiple coats with sufficient curing between coats to allow solvent and byproducts of the curing process to escape. Enough coats are built up to completely cover the conductors, but to barely cover the via pillars. Short etch or even mechanical lapping is sufficient to uncover the top surface of the via pillars. While this method results in a planar surface, it involves a large number of steps and, again, cannot be used where an overlay layer must bridge a gap between two chips.
In addition to the problems associated with the approaches described above for providing via holes, it is noted that these processes cannot be achieved without the use of wet processing; that is, wet chemistry must be employed for developing a photoresist for etching of the mask or for the plating of the via areas. A distinguishing characteristic of the disclosed invention is that it is achieved using a plasma etch, which is a dry process.
The use of lasers for drilling holes is another method employed in the prior art to provide vias, Typically, a laser is used in a pulsed mode to evaporate polymer material wherever the laser energy is concentrated. Very short pulses heat the material to the point that it vaporizes. This approach, however, is not satisfactory for providing via holes in the circumstances contemplated herein. First, in such methods, the underlying pads may be damaged by energy which is sufficient to vaporize the polymer. It is unacceptable to damage the underlying pads. Second, the process is relatively slow in that several pulses are required. In an interconnect system, a large number of holes is required so that slow processes are again unacceptable.
The polymer film may also be provided with holes by the process described in copending and commonly assigned patent application Ser. No. 912,455 (RD-17428) in which a laser is focussed on a polymer to sensitize it to the extent that it can be selectively removed in a plasma etching process. In such a process, however, the sensitization of the polymer requires that radiation within a given exposure window be applied and this may be difficult to control because of variations in laser inter-pulse intensity, polymer absorption, etc.